Immunomodulation by periodontopathic bacteria has been implicated in the pathogenesis of inflammatory periodontal diseases. A novel class of microbial-derived T cell mitogens, referred to as superantigens (SAg), has recently been described. SAg are unique in that they activate and expand large subsets of T cells in an antigen-independent manner, and these subsets can be identified with reagents that discriminate between different subtypes (families) of the variable domain of T cell antigen receptor (TCR Vbeta. SAg are believed to cause immune dysfunction in a number of diseases by large-scale T cell activation, leading to: l) elaboration of overwhelmingly high levels of some cytokines, and depression of other cytokines, thereby disrupting normal immunoregulatory homeostasis and mediating tissue injury; 2) polyclonal B cell activation, 3) activation and expansion of quiescent autoreactive T cells, and 4) rendering the directed immune response less efficient. In preliminary studies, we have demonstrated that: P. gingivalis and A.actinomycetemcomitans have SAg properties in vitro and 2) in most periodontal patients, there is an overrepresentation of some subsets of gingival T cells, marked by their TCR Vbeta region expression, which is one of the hallmarks of in vivo SAg stimulation of T cells. In some patients, a few TCR Vbeta families comprised nearly 50% of all gingival T cells suggesting that SAg constitute a major pathway of T cell activation and expansion. This skewing of T cell subsets was particularly striking among activated T cells, suggesting in vivo stimulation of those T cells (most likely by SAg). Hence, we hypothesize that some of the putative periodontopathic bacteria produce superantigens that activate and expand a large number of T cells in a antigen-independent fashion. Moreover, superantigen-expanded T cells are present in periodontitis sites. To investigate our hypothesis, we have developed specific aims to: 1) determine whether P. gingivalis and A.actinomycetemcomitans can indeed qualify as SAg. This will be accomplished by investigating key properties that distinguish SAg from conventional Ag, utilizing in vitro murine and human systems. 2) Evaluate the existence of in vivo SAg-stimulated T cell subsets in periodontitis sites and 3) purify and characterize P. gingivalis- and A. actinomycetemcomitans -associated SAg. If our hypothesis is confirmed, it has important implications on the pathogenesis of periodontitis, involving initial Ag-independent activation and expansion of T cells, polyclonal B-cell activation, dysregulated cytokine release and generation of autoreactive T and B cells. Furthermore, identifying domains of the TCR molecules and the bacterial components that interact with them, lays the foundation for devising new immunoerapeutic approaches involving more specific targeting of these molecules.